vfio/mtty: Enable migration support

The mtty driver exposes a PCI serial device to userspace and therefore
makes an easy target for a sample device supporting migration.  The device
does not make use of DMA, therefore we can easily claim support for the
migration P2P states, as well as dirty logging.  This implementation also
makes use of PRE_COPY support in order to provide migration stream
compatibility testing, which should generally be considered good practice.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20231016224736.2575718-3-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>